"This report is going to be particularly unwelcome for anyone in their early 40s, as they're now likely to see their state pension age pushed back another year," said Tom McPhail, head of retirement at Hargreaves Lansdown.

"For those in their 30s and younger, it reinforces the expectation of a state pension from age 70, which means an extra two years of work."

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In an extreme scenario, experts from the Government Actuary's Department (GAD) said the state pension age could be raised as high as 70 as soon as 2054.

Under existing plans, the state pension age is due to rise to 68 for those born after 1978.

The "extreme" scenario involves an assumption that people spend 32% of their adult life in retirement. The conventional assumption until now has been that people will spend 33.3% of their lives in retirement.

In the worst-case situation, the GAD calculations also suggest that the change in the retirement age from 67 to 68 could be pulled forward by as much as 16 years.

So while that increase is not due to happen until 2044, it could be brought in as soon as 2028, affecting those now in their late 50s.

'Extra year'

Former pensions minister Steve Webb was highly critical of the GAD's scenario.

"This is not what parliament voted for and is clearly driven by the Treasury. It is one thing asking people to work longer to make pensions affordable, but it is another to hike up pension ages because the Treasury sees it as an easy way to raise money," he said.

However, the other report, by the former CBI chief John Cridland, foresees more modest changes.

He recommends bringing the change from 67 to 68 forward by seven years, from 2046 to 2039. That would mean anyone currently under the age of 45 having to work an extra year.

The changes are due to be phased in gradually, over a two-year period in each case.

In addition Mr Cridland said there should be no up-rating from 68 to 69 before 2047 at the earliest, and that the pension age should never rise by more than one year in each ten-year period.

He also suggests that the so-called triple lock be ended in the next parliament.

Up to now the triple lock has guaranteed that the state pension rises each year by inflation, earnings or 2.5%, whichever is the highest.

However, by linking the rise in pension payouts to earnings alone, the bill for pensions would fall from 6.7% of GDP to 5.9% of GDP by 2066.